it's not an issue, so don't panic.
The 28-day cut off date is based on the date you submit the online application and pay for the visa. It is not based on biometrics date.
If you have done this today, then you have nothing to worry about, because your latest payslip, bank statement and the employer letter only have to be dated within 28 days of today's date.
The whole application system has changed in the last few months now though and nothing is mailed anymore .
As far as I know, you do not need to part with your passport, as I believe you now upload a copy of it instead, but you do take it to the biometrics anyway (if you did still mail everything, you would have been without your passport for up to 2 months)
You now:
- scan and upload all your documents (or pay for a biometrics appointment where they will scan them for you)
- attend the biometrics appointment (taking your documents with you just in case they need to rescan them)
- wait for the visa to be processed